Method for correcting weft distortions in woven webs

ABSTRACT

Bow distortions of the weft threads of woven webs are removed by normally operating a tenter frame at a slightly faster speed than a roll feed drawing the web forwardly from the tenter, thereby intentionally inducing a lagging bow in the web. Bow detection means are connected to momentarily slow the speed of the tenter to less than that of the roll feed when the lagging bow attains an acceptable maximum, allowing the weft threads to return to a straight configuration and eventually toward a leading bow of maximum acceptable magnitude, whereupon the tenter is caused to resume its normal slightly faster speed.

Hannaway [451 Jan. 25, 1972 [54] METHOD FOR CORRECTING WEFI DISTORTIONS IN WOVEN WEBS [72] Inventor: Jolhn H. Hannaway, Pawtucket, R.1.

Mount Hope Machine Company, Incorporated, Taunton, Mass.

[22] Filed: Nov. 6, 1969 [21] Appl. No.: 874,503

I 73] Assignee:

2,311,406 2/1943 Mansfield et al. ....26/52 2,968,856 1/1961 Allen ....26/51.5

2,219,213 10/1940 Swain ..26/51.5 3,296,676 Macomson OTHER PUBLICATIONS Electronic Straighteners Use Two Different Systems," by D. Eldred, Textile World, October, 1947.

Coltron Solid State Fabric Straighteners, Coltron Industries, Inc., Bulletin COL-100. received in Patent Office 12/12/1967.

Primary ExaminerRobert R. Mackey AttorneyRich & Ericson [57] ABSTRACT Bow distortions of the weft threads of woven webs are removed by normally operating a tenter frame at a slightly faster speed than a roll feed drawing the web forwardly from the tenter, thereby intentionally inducing a lagging bow in the web. Bow detection means are connected to momentarily slow the speed of the tenter to lessthan that of the roll feed when the lagging bow attains an acceptable maximum, allowing the weft threads to return to a straight configuration and eventually toward a leading bow of maximum acceptable magnitude, whereupon the tenter is caused to resume its normal slightly faster speed.

3 Claims, 4 Drawing Figures PATENTEU JAN25 I972 SHEET 1 [If 2 38 a JoPaZz/HH ea ray ggm mu z W PATENTED JAHZS I972 sum-aura METHOD FOR CORRECTING WEF'I DISTOR'IIONS IN WOVEN WEISS BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION In processes such as laminating and bonding woven webs with backing material, it is essential to ensure that the weft threads are straight and perpendicular to the warp threads as the cloth feeds into the processing stage. This is especially critical and also difficult when patterned materials such as plaids and stripes are being processed. Any noticable weft distortion results in the production of unsaleable material.

It is the conventional practice in a bonding or laminating range to feed the web over a roll feed, which is driven to feed the web through the processing stage. The web is framed in a differential tenter before it reaches the roll feed. The chainmounted clips or pins of the tenter engage the opposite selvages of the cloth, and an operator has independent manual control of the speeds of each chain. His observations of the condition of the weft threads, if his eye is sufficiently sharp and alert, enable him to correct weft distortions by regulating the speeds of the tenter chains individually with respect to one another, or jointly with respect to the speed of the roll feed. He can correct a skew condition, in which the ends of the weft threads at one selvage of the cloth trail behind those at the other, by speeding up the chain holding the trailing edge. A bowed weft, in which the central portions of the weft threads lead or lag the ends, can be straightened by speeding up or slowing down both tenter chains in unison. This is a very difficult operation to carry out with any accuracy, especially when a combination of bow and skew distortions appears, as occurs very frequently. Even the most skilled operators cannot control the tenter accurately if the cloth is fed at more than a moderate rate of speed.

The demand for increased production rates has consequently led to eflorts to automate weft-straightening by means of a differential tenter frame. The assignee of this application has successfully accomplished this by using weft detectors of the kind described and claimed in the U.S. Pat. No. 3,350,933, issued Nov. 7, 1967 to Leo J. Smith, and entitled Weft-Detecting Method and Apparatus." Control signals generated by these detectors have been applied to increase or decrease the speed of one or both tenter chains with respect to the speed of the roll feed, as required to correct either leading or lagging bow distortions, or skewing of the weft threads to lead at the right or left selvages of the cloth. This has been done with varying degrees of effectiveness by the alternative expedients of using the control signals either to operate pilot motors which affect the frequencies of motor-generators energizing variable-speed tenter driver motors, or to adjust a variable-speed belt drive for the tenter chains.

Drawbacks of these prior methods of automatic control include a tendency of such systems to hunt or fluctuate constantly between unacceptably large amounts of leading and lagging bow. Overcorrection is unavoidable, since a corrective rate of change of speed of the tenter rails is not produced until the maximum tolerated bow in either sense is attained. Because this rate of change of speed is finite, and is in fact usually rather slow, the speed does not change instantaneously, so that the bow distortion continues to increase beyond the acceptable limit until such time as the speed of the rails has changed enough to start to reduce the bow error. Moreover, the resulting continual operation of the control system leads to relatively rapid wear of the system and requires rather frequent maintenance and adjustment. The equipment required is also somewhat complicated and therefore relatively costly to purchase and install.

It is the primary object of the present invention to provide an improved method and apparatus for correcting bow distortions in the weft threads of a woven web, which is less subject to hunting and overcorrection of distortions than previously known systems and methods. Further objects and advantages of the system will appear as the following description proceeds.

I warp the web partially about one or more draw rolls driven at a uniform, predetermined speed. This establishes a web speed in the region of the roll feed which is uniform for all warp elements, i.e., the speed is constant at all points across the width of the web. In a region of the web which is spaced away from the draw roll feed in the direction of the source of the web, I engage the selvages on the clips or pins of a differential tenter frame.

According to the invention, the tenter chains are normally driven at a speed which is slightly greater than the predetermined speed of the roll feed; the result is a tendency for the weft threads gradually to assume a lagging bow, with the central portions lagging behind the ends of the weft threads at the selvages of the cloth. When the maximum acceptable amount of lagging bow is reached, a control system reduces the tenter chain speed to somewhat less than the speed of the roll feed for a fixed, brief interval. This permits the selvages to slow relative to the center of the web, and causes the weft distortion to change from a lagging toward a leading bow configuration. After an interval of this reduction of chain speed, the control system is inactivated and ceases to produce speedreducing pulses, and the chains revert to their original higher speed. The control system is blocked in its inactive state for a further interval to allow a lagging bow to develop once again, but is reactivated so that it can slow down the tenter chains before an excessive amount of lagging bow develops.

This method of bow control requires only one condition or control point to be detected for control actuation; this the presence or absence of a lagging bow corresponding to a maximum tolerance limit.

Both the intervals of tenter chain speed reduction and of control inactivity are preferably adjustable to suit the operating conditions. The former should be sufiicient to allow the weft threads to attain a slightly leading how. The latter should permit only a similar permissable amount of lagging bow to appear before the control system is reactivated.

It is contemplated within the broader aspects of the invention that the tenter chains might normally be run at a speed slightly less, instead of greater, than the speed of the roll feed. This would induce an intentional leading bow in place of a lagging one,and the limiting configuration of the weft threads would be a permissable limit of leading bow. This mode of practice is not preferred, however, since the pulses would tend to cause excess material to accumulate between the tenter frame and the roll feed, possibly resulting in loss of control of the web because of excess slackness.

DESCRIPTION OF THE DRAWINGS While the specification concludes with claims particularly pointing out the subject matter which I regard as my invention, it is believed that a clearer understanding may be gained from the following detailed description of preferred embodiment, referring to the accompanying drawings, in which:

FIG. 1 is a schematic plan view showing a first form of the invention applied to a bonding operation;

FIG. 2 is a schematic sectional view in side elevation taken along line 2-2 in FIG. 1, looking in the direction of the arrows;

FIG. 3 is a schematic plan view of a portion of the apparatus, showing several types of weft distortions for purposes of illustrating the mode of operation of the apparatus; and

FIG. 4 is a fragmentary schematic plan view showing a modification of the apparatus.

Referring to FIGS. 1 and 2, a first form of apparatus is illus trated as it may be applied to a process of bonding an indefinite length of woven web material 10 with a backing sheet of lining material, such as nylon tricot for example. The web 10 is fed in the direction shown by the arrows by driving the curing drum 86 of a bonding range, about which it is partially wrapped, and is supplied in a stabilized flat condition from suitable tensioning devices, stabilizing rolls, expanders, or other conventional apparatus (not shown) for preparing the cloth for straightening and framing in a difierential tenter frame 16. The tenter has a pair of chains 12 trained around idler sprockets l4 and drive sprockets 18, the latter being mounted on independent shafts 34 so that the chains may be driven at different speeds. The opposite selvages of the web are engaged with pins 13 carried by the links of the tenter chains. The web ordinarily passes from the tenter frame 16 over a conventional expander, illustrated as a curved roll expander90, and a suitable wrap angle is established by idler rolls 88. A coating of bonding adhesive is applied by rolls 92 to the upper surface of the web 10, which is then bonded by rolls 96 with backing material 95 drawn from a supply spool 94. Idler rolls 98 wrap the laminated material through a suitable are about the curing drum 86, which is heated to set the bonding adhesive. The application of the weft-straightening method is not limited to a bonding operation, which is shown only for purposes of illustration; it may be used in many processes involving a roll feed supplied with straightened cloth by a tenter.

The shaft 85 of the curing drum is driven at a uniform linear surface velocity by an electric motor 83, through a speedreducing drive comprising a chain 75 trained around sprockets 76 and 78; a jack shaft 74, and a chain 84 trained around sprockets 80 and 82. In the form shown in FIGS. 1 and 2, the tenter chains 12 are arranged to be driven at adjustable speeds by the range drive motor 83. A shaft 65 carrying a normally engaged electrically operated clutch 66 is driven from the jack shaft 74 by a chain 70 trained around sprockets 72 and 68, and normally drives a shaft 50 through the clutch and a chain 62, trained on sprockets 64 and 60.

A reduced speed of the shaft 50 is obtainable by deenergizing and thereby disengaging the clutch 66. The shaft 65 also carries a sprocket 56, driving an overrunning sprag clutch 58 through a chain 54 and sprocket 52. The clutch 58 is arranged to freewheel when the clutch 66 is energized and engaged to drive the shaft 50 at a normally higher speed; but deenergization and disengagement of the clutch 66 bypasses the drive through the slower speed sprocket train 56 and 52 and the overrunning clutch 58, to drive the shaft 50 at a reduced speed. The speed ratio between the alternate drive trains is not greatly different from unity, depending on the judgment of the design engineer as to the requirements of a particular installation, but will not normally be much more than 100:99. This is done to avoid a condition in which a too-much-reduced tenter speed would stretch the selvages between the tenter and the curing drum 86 to the point of damage. Likewise, it is necessary to avoid a too-much-increased tenter speed which would produce enough slack to cause detector wheels 24, which will be further described hereinafter, to lose control.

The shaft 50 drives an adjustable speed reducer 46, which may be of a well-known belt-and-cone type such'as the Reeves Drive, and which has a manual adjustment wheel 48. This permits the precise linear speed ratio between the bonding range 86 and the tenter chains 12 to be varied as necessary for the best treatment of the particular web material being processed at any time.

In order that the speeds of the tenter chains 12 may be varied individually for the purpose of straightening skew distortions in the wefi of the cloth, the drive shaft 26 has two alternate driving connections of different speed ratios with each of the chain drive sprockets 18. The normal driving connection is made by chains 30 trained around sprockets 28, 32, the latter being connected with the shafts 34 of sprockets 18 by means of overrunning sprag clutches 36. An increase in the speed of either sprocket 18 can be obtained by energizing a corresponding one of a pair of normally disengaged, electrically operable clutches 40. This drives one of the shafts 34 through a chain 42 trained around an associated pair of sprockets 38, 44, which have a higher speed ratio than the sprocket pairs 28, 32. When one of the clutches 40 is engaged,

the corresponding sprag clutch 36 overruns so that the shaft 34 may rotate with increased speed.

The configurations of weft threads in the region of the cloth passing over the tenter frame are detected automatically by a pair of detector heads 22, each having a swiveling caster wheel 24 riding on the cloth. The detection system is preferably designed in accordance with US. Pat. No. 3,350,933 issued Nov. 7, 1967 to Leo J. Smith, entitled Weft-Detecting Method and Apparatus," and assigned to the assignee of the present application. Accordingly, two detector heads 22 are mounted upon a supporting structure including a transverse rod 20, and engage the cloth on opposite sides of its centerline. A skewed condition of the weft threads is reflected by both caster wheels 24 turning in the same angular sense from a normal running position parallel to the length of the cloth, while a bowed condition is indicated by the wheels turning in opposite angular directions from their normal positions.

An example of the operation of the detectors is shown in FIG. 3, in which the weft threads 11 of the web 10 are shown in their normal right-angular relation to the warp threads 13 in region A, in a bow distortion in region B, and in a skew distortion in region C. The transition from a pure bow to a pure skew results in an overlap of regions B and C, where a compound distortion appears; this is frequently encountered in actual practice.

The caster wheels 24 are pressed into the plane of the web 10, and produce local depressions which cause the warp threads to pass under the wheels at slight angles to the length of the web if the weft threads are distorted. The wheels 24 follow the warp threads, and their angular positions are therefore a measure of the weft thread configurations. Transducers in the heads 22 produce output signals which reflect the angular positions of the wheels, and these signals are applied by electrical leads 102 in a bridge circuit A mounted in a control panel 100.

As more fully described by the aforementioned US. Pat. No. 3,350,993, the circuit A simultaneously provides output signals representing the algebraic sum of the angular displacements of the wheels 24, indicating skew distortions, and the algebraic difference of these angular displacements, indicating bow distortions. The sense of a skew distortion, that is a weft leading at the right or left edge of the web, and the sense of a bow distortion, whether involving central portions of the weft threads leading or lagging their ends, are determined by including conventional phase-discrimination means in the circuit A.

In the event that there is a skew distortion in which the right-hand ends of the weft threads are leading the left-hand ends, the circuit A applies a control signal to a relay R1, closing a normally open relay contact IR] and energizing the clutch 40 at the left side of the tenter frame 16 through a lead 106, and thereby increasing the speed of the left-hand tenter chain 12 to a faster rate than the right-hand chain. This tends to cause the left selvage to catch up with the right selvage, thereby correcting the skew by bringing the weft threads into perpendicularity with with the length of the cloth. An adjustable timing circuit Tl limits the duration of the energizing signal to the left-hand clutch 40; and also sets a limit to the frequency of energization, by establishing a time interval during which no energizing signal can be delivered, for the purpose of curtailing hunting or continuous cycling of the control system. A service engineer may set the duration and frequency of skew signal response for the optimum corrective action in any given installation.

Similarly, if a skew distortion occurs in which the left-hand ends of the weft threads lead the right-hand ends, such as is illustrated in region C of FIG. 3, the circuit A will apply a control signal to a relay R2 through the timing circuit T1. This closes a normally open relay contact 1R2 to energize a lead 107, energizing the clutch 40 at the right side of the tenter frame 16 and thus increasing the speed of the right-hand tenter chain 12 to a faster rate than the left-hand chain. This causes the right-hand selvage to overtake the left-hand selvage, correcting the skew distortion.

The algebraic difierence of the detector signals indicates how distortion, and a net output signal whose phase represents a lagging bow, i.e., a bow as illustrated in region B of FIG. 3, in

which the central portion of the weft threads lags behind their ends, is applied through a pulse-timing circuit T2 to a relay R3. This signal opens a normally closed relay contact 1R3, deenergizing a lead 104 which is normally energized, and causing the clutch 66 to disengage. The drive of the shaft 50 is transferred from the chain 62 to the chain 54 by the ensuing engagement of the overrunning clutch 58, decreasing the speed of the shaft 50, variable-speed drive 46, shaft 26, shafts 34, and both drive sprockets 18. The drive train value is so designed that the linear velocity of the tenter chains 12 is then somewhat less than the linear velocity of the curing drum 86. This allows the selvages of the web to lag relative to the center, so that the weft threads tend to shift toward a leading bow from their previous lagging bow.

The timing circuit T2 is designed and adjusted to limit the period and frequency of speed-reduction of the tenter chains 12 so that the weft will at no time assume more than a small leading bow distortion, but so that control actuation will not have a continuously cycling or hunting characteristic. At the conclusion of the preset period of actuation, the relay R3 is deenergized, allowing the contact 1R3 to reclose and thus energize and reengage the clutch 66. This accelerates the chains 12 to their normal speed, and tends to draw the weft threads once again into a lagging bow configuration.

The frequency of operation of the relay R3 by the timing circuit T2 is set to obtain sufiiciently short intervals to avoid such a large degree of lagging bow in the weft threads that the resulting laminated web would not satisfy production tolerances. A maximum lag in the central portions of the weft threads of one-quarter inch behind the ends would, for example, normally fall well within the acceptable tolerance for a web a few feet in width. At the same time, the duration of closure of the relay R3 is set at a sufficiently short period to avoid the occurrence of any leading bow condition in excess of the same tolerance.

The result is that the weft threads cycle slowly between a slightly lagging but acceptable bow, and an approximately equal leading bow. There is but one limiting condition for control actuation; this is the presence or absence of a lagging bow of the maximum allowable amount.

Also, control response is limited to fixed brief intervals occurring with a predetermined maximum frequency. Consequently, the improved apparatus and method of bow correction afford a great reduction in hunting or continuous cycling operation of the control system.

An alternative arrangement would be to provide means for producing three driving speeds for the tenter chains: slightly 'overspeed, synchronous, and slightly underspeed with respect to the curing drum 86. Such a system, while possibly acceptable in stability characteristics, would be substantially more complicated and expensive, both in the tenter drive means and in the control system.

The speeds of the sprockets 18 may be adjusted within a moderate range by the hand crank 48 as necessary to insure the development of a lagging bow when the tenter chains are running at their faster rate. It should also be noted that the process of skew correction by energizing one of the clutches 40 superimposes an addendum on the speed of one or the other of the sprockets 18, so that compound distortions involving both bow and skew, as illustrated for example in the region C of FIG. 3, can be corrected by simultaneous different speed adjustments of the two sprockets 18.

A modification of the apparatus is shown in FIG. 4, from which some unchanged portions of the system are omitted, and like parts are numbered the same as those of FIGS. 1 and 2. In this case the laminating range drive motor 83 is not used to drive the tenter chains 12, and this function is performed alternatively by fixed-speed drive motors 120 or 122. The motor 120 normally drives the shaft 50 at a higher rate of speed through a chain 118 and sprockets 114 and 116, delivering a linear velocity to the tenter chains 12 which slightly exceeds the linear velocity of the laminating range 86 and therefore tends to induce a lagging bow in the web 10. The motor 120 is energized by a normally closed relay contact 1R3 while the relay R3 remains deenergized, in the absence of a bow-in-.

dicating signal from the bridge circuit A and time-delay circuit T2.

The motor 122 operates at the same speed as the motor 120, but is connected to the shaft 50 by a chain 112 and sprockets 110, 108 which deliver a somewhat slower drive train ratio than the sprockets 116, 114, and are selected to drive the tenter chains 12 at a somewhat slower linear velocity than the curing drum 86. Application of a bow-indicating signal to ,the relay R3 by the circuits A and T2 closes a normally open relay contact 2R3 to energize the motor 122, while opening the contact 1R3 to deenergize the motor 120, thereby retarding the speed of the tenter chains to allow the weft threads of the web to shift from a lagging bow toward a leading bow. The operation of this system is similar to that of FIGS. 1 and 2, and will be readily understood from the foregoing description of that system.

It will be perceived from the discussion of the illustrated apparatus that my improved method of weft-straightening entails the steps of feeding one region of the web at a speed which is uniform for each of its longitudinal warp elements, while gripping and feeding only the selvages, in another region closer to the webs source, at a speed which is normally somewhat faster. This tends to induce a lagging bow in the weft threads. My method also requires that the extent of this bowing shall be determined, and that the rate of feeding of the selvages in the second region shall be reduced temporarily below the velocity of the first region when the weft threads have become bowed to an extent that does not exceed acceptable limits. This causes the weft threads to straighten and then assume a leading how; I limit the reduction of selvage speed to a brief interval, occurring with no more than a given frequency, so that the weft does not stray beyond an acceptable amount of leading bow. The optimum interval and frequency can be readily determined by experiment and may vary substantially under different circumstances, but the interval of speed reduction may typically extend from one to a few seconds at most, and occur with a frequency of one such interval in every 5 or 10 seconds. I

It is alternatively possible normally to drive the tenter chains at a speed less than that of the roll feed, tending to induce a leading instead of a lagging bow, but this is not preferable because during the period of control pulsing, the web will tend to accumulate between the tenter and the roll feed, resulting in possible loss of control of the web.

What I claim is: l. The method of correcting bow distortions in the weft threads of a longitudinally travelling woven web by means of a tenter having opposed chains, which comprises the steps of:

feeding the web longitudinally by gripping a first region of the web and advancing this region at a predetermined speed which is uniform across the full width of the web;

gripping opposed selvage portions of the web by means of the opposed chains of the tenter along a substantial length thereof in a second region of the web spaced away from said first region toward the source of the web, while feeding the tenter chains and the gripped selvage portions forwardly toward said first region at a speed greater than said predetermined speed, thereby tending to how the weft threads in said second region in a sense such that the portions of the weft threads near the center of the web will lag behind the portions of the weft threads near the selvages of the web;

determining the extent of bowing of the weft threads within said second region;

temporarily reducing the feeding rate of the tenter chains and the gripped selvage portions in said second region to a lesser speed than said predetermined speed for a predetermined interval in the event that the weft threads in said second region become bowed to a predetermined extent,

resuming the feeding of the tenter chains and the selvage portions in said second region at said greater speed after said predetermined interval;

and repetitively alternating the feeding rate of the tenter chains and the gripped selvage portions in said second region between said greater speed and said lesser speed in accordance with the preceding steps.

2. The method recited in claim 1, in which the step of temporarily changing the feed rate of the tenter chains and the selvage portions in said second region to a lesser speed than said predetermined speed is repeated with no more than a fixed maximum frequency, and the tenter chains and the selvage portions in said second region are fed at said greater speed between successive ones of said predetermined intervals.

3. The method of correcting bow distortions in the weft threads of a longitudinally travelling woven web by means of a tenter having opposed chains, which comprises the steps of:

feeding the web longitudinally by gripping a first region of the web and advancing this region at a predetermined speed which is uniform across the full width of the web; gripping opposed selvage portions of the web by means of the opposed chains of the tenter along a substantial length thereof in a second region of the web spaced away from said first region toward the source of theweb, while feeding the tenter chains and the gripped selvage portions forwardly toward said first region at a first speed different from said predetermined speed, thereby tending to bow the weft threads in said second region;

determining the extent of bowing of the weft threads within said second region;

temporarily changing the feed rate of the tenter chains and the gripped selvage portions in said second region for a predetermined interval to a second speed differing from said predetermined speed in an opposite algebraic sense from said first speed, in the event that the weft threads in said second region become bowed to a predetermined extent, I

resuming the feeding of the tenter chains and the selvage portions in said second region at said first speed after said predetermined interval;

and repetitively alternating the feeding rate of the tenter chains and the gripped selvage portions in said second region between said first speed and said second speed in accordance with the preceding steps. 

1. The method of correcting bow distortions in the weft threads of a longitudinally travelling woven web by means of a tenter having opposed chains, which comprises the steps of: feeding the web longitudinally by gripping a first region of the web and advancing this region at a predetermined speed which is uniform across the full width of the web; gripping opposed selvage portions of the web by means of the opposed chains of the tenter along a substantial length thereof in a second region of the web spaced away from said first region toward the source of the web, while feeding the tenter chains and the gripped selvage portions forwardly toward said first region at a speed greater than said predetermined speed, thereby tending to bow the weft threads in said second region in a sense such that the portions of the weft threads near the center of the web will lag behind the portions of the weft threads near the selvages of the web; determining the extent of bowing of the weft threads within said second region; temporarily reducing the feeding rate of the tenter chains and the gripped selvage portions in said second region to a lesser speed than said predetermined speed for a predetermined interval in the event that the weft threads in said second region become bowed to a predetermined extent, resuming the feeding of the tenter chains and the selvage portions in said second region at said greater speed after said predetermined interval; and repetitively alternating the feeding rate of the tenter chains and the gripped selvage portions in said second region between said greater speed and said lesser speed in accordance with the preceding steps.
 2. The method recited in claim 1, in which the step of temporarily changing the feed rate of the tenter chains and the selvage portions in said second region to a lesser speed than said predetermined speed is repeated with no more than a fixed maximum frequency, and the tenter chains and the selvage portions in said second region are fed at said greater speed between successive ones of said predetermined intervals.
 3. The method of correcting bow distortions in the weft threads of a longitudinally travelling woven web by means of a tenter having opposed chains, which comprises the steps of: feeding the web longitudinally by gripping a first region of the web and advancing this region at a predetermined speed which is uniform across the full width of the web; gripping opposed selvage portions of the web by means of the opposed chains of the tenter along a substantial length thereof in a second region of the web spaced away from said first region toward the source of the web, while feeding the tenter chains and the gripped selvage portions forwardly toward said first region at a first speed different from said predetermined speed, thereby tending to bow the weft threads in said second region; determining the extent of bowing of the weft threads within said second region; temporarily changing the feed rate of the tenter chains and the gripped selvage portions in said second region for a predetermined interval to a second speed differing from said predetermined speed in an opposite algebraic sense from said first speed, in the event that the weft threads in said second region become bowed to a predetermined extent, resuming the feeding of the tenter chains and the selvage portions in said second region at said first speed after said predetermined interval; and repetitively alternating the feeding rate of the tenter chains and the gripped selvage portions in said second region between said first speed and said second speed in accordance with the preceding steps. 